It is common in the art with devices that receive an input power and in response to the received input power transmit an output power. The ratio between the output power and the input power is normally referred to as (power) gain. The gain is often frequency dependent. This may be desirable in some instances but in most cases undesirable.
A power amplifier is a device which is used for amplifying a signal power. The power amplifier is used in many technical applications, e.g. broadcast radio and TV, wireless communications (such as cellular telephony), cable TV, hi-fi audio equipment et cetera. For the power amplifier, the frequency dependence of the gain is often an important feature to consider.
The construction of the power amplifier is often based on transistor technology, and the bipolar transistor is probably the most frequently used transistor element in power amplifiers. However, vacuum tubes, once considered obsolete, are still used today for some applications. The power amplifier can be built with discrete components or with components arranged on an integrated circuit.
The power amplifier is normally designed to provide power amplification for signals in a predetermined operating frequency range. However, it is difficult to obtain a uniform (constant) gain over the whole of the operating range, especially for broadband ranges. Normally, the gain of the power amplifier decreases with increasing frequency.
The frequency dependence of the gain of the power amplifier is troublesome in many technical applications. One such application is the so-called feed forward amplifying circuit. The feed forward amplifying circuit includes a main power amplifier, which operates in a non-linear mode. The feed forward amplifying circuit further includes a feed forward loop. The feed forward loop includes means for generating an indication signal which is indicative of distortion products due to the non-linearity of the main amplifier. The feed forward loop further includes an (linear) error amplifier which generates an error signal by amplifying the indication signal. The error amplifier is set so that the error signal corresponds as closely as possible to the distortion products generated in the main amplifier. The error signal is subtracted from an output signal from the main amplifier, thereby suppressing the distortion products. However, if the suppression of the distortion products is to be efficient, the gains of the main and error amplifier must not vary with frequency to any considerable extent.
Another application, where the frequency dependence of the gain is troublesome, is cable TV transmissions, where the effect of lower gain at higher frequencies may lead to a loss of picture detail and colour saturation.
Naturally, the power amplifier is not the only device for which the frequency dependence of the gain can be troublesome. The frequency dependency is also troublesome, for example, for couplers, transmission cables, stiplines, microstrips, mixers and radio frequency equipment in general.
U.S. Pat. No. 5,656,973 discloses an amplifying circuit, which includes an MMIC (Monolithic Microwave Integrated Circuit) power amplifier in combination with a compensation circuit. The compensation circuit is designed to have a frequency response which counteracts a frequency dependence of gain of the power amplifier. The compensation circuit is a resonant band-pass filter including a resistor, an inductor and a capacitor. This compensation circuit, however, has some drawbacks. The gain compensation associated with the compensation circuit cannot be easily tuned. A big “spread” in the actual compensation can be expected due to variations in component values. Furthermore, the compensation circuit is not suitable for providing minor corrections (±0.1 dB or so).
U.S. Pat. No. 5,280,346 discloses an equalising amplifying circuit, which is used for compensating for frequency dependent losses in a television cable. The circuit includes an equalising network and a variable amplifier for generating a correction signal, which compensates for the frequency dependence of the cable. In order to make the circuit more useful for different lengths of cable, the circuit includes a positive feedback loop having an attenuator. Because of the positive feedback, the circuit can be used with different lengths of cable by appropriately controlling the variable amplifier. The circuit is, however, rather complicated and expensive. Another drawback is that the circuit can only be used for lower frequencies (base band) and not for higher frequencies (radio frequency).